Back when I first got serious about science back in my late teens and early 20's, time travel was my top choice of research area -- but after a little while I decided that AI, my second choice, was more likely to be achievable within my lifetime. (EVEN FURTHER DIGRESSION: My third choice was working toward human immortality via biology -- which I'm working on in the background now via my work with Biomind LLC -- but I backburnered that one because I don't enjoy biology personally nearly as much as physics or computer science. To me, it seems that biological immortality will be made possible via a combination of many relatively small insights and leaps -- "big science" like one sees in contemporary biology -- whereas time travel and AI seem more amenable to huge revolutionary insights ... this is one reason the latter interest me more than biology ... the other being that CS and physics have a mathematical elegance that appeals to me, and that biology lacks....)

I learned in the mid-80's, when studying general relativity theory in grad school at NYU, that modern physics deems time travel possible -- but difficult to achieve. Basically, it makes time travel into an engineering problem, but one that would seem to probably require engineering on the scale of making weird configurations of exotic forms of matter and energy ("exotic" meaning physically possible to produce, but incredibly difficult and/or expensive to do so using current technologies). Do-able, but probably not this decade....

Much later I read Kip Thorne's book "Black Holes and Time Warps", which reviews general relativity and its implications as regards time travel (along with other topics), and a host of other related papers, some of which are reviewed and referenced here.

(A much more ridiculous, though amusing, book on time travel is J.H. Brennan's book, "Time Travel: A New Perspective." Brennan gives you practical instructions on how to travel through time. Recommended only for entertainment value. One of the reviewers on amazon.com complains that the methods are inadequate because they can't be practiced by individuals acting alone, they require that time travel be a group activity!)

Anyway, the article Dave forwarded is brief and the bulk of it goes as follows:

The laws of physics seem to permit time travel, and with it, paradoxical situations such as the possibility that people could go back in time to prevent their own birth. But it turns out that such paradoxes may be ruled out by the weirdness inherent in laws of quantum physics.

Some solutions to the equations of Einstein's general theory of relativity lead to situations in which space-time curves back on itself, theoretically allowing travellers to loop back in time and meet younger versions of themselves. Because such time travel sets up paradoxes, many researchers suspect that some physical constraints must make time travel impossible. Now, physicists Daniel Greenberger of the City University of New York and Karl Svozil of the Vienna University of Technology in Austria have shown that the most basic features of quantum theory may ensure that time travellers could never alter the past, even if they are able to go back in time.

The constraint arises from a quantum object's ability to behave like a wave. Quantum objects split their existence into multiple component waves, each following a distinct path through space-time. Ultimately, an object is usually most likely to end up in places where its component waves recombine, or "interfere", constructively, with the peaks and troughs of the waves lined up, say. The object is unlikely to be in places where the components interfere destructively, and cancel each other out.

Quantum theory allows time travel because nothing prevents the waves from going back in time. When Greenberger and Svozil analysed what happens when these component waves flow into the past, they found that the paradoxes implied by Einstein's equations never arise. Waves that travel back in time interfere destructively, thus preventing anything from happening differently from that which has already taken place. "If you travel into the past quantum mechanically, you would only see those alternatives consistent with the world you left behind you," says Greenberger.

Interesting... huh?

What this suggests is that, perhaps, time travel is quite possible and the reason that it seems paradoxical to us is because of our illusion of free will.

I.e. since we think we have free will, we don't like to think that if we go back in time we are constrained to do things consistent with presently observed reality...

I am reminded of Ouspensky's classic novel "The Strange Life of Ivan Osokin" -- where the main character Osokin convinces a magician to send him back in time to live through his life all over again... he's hoping to make his life better, by not making as many stupid decisions as he did the previous time around.

But the punchline is, while living his life over again, Osokin winds up making the same stupid decisions he did before. He just can't help himself -- he finds himself irresistably drawn to make the same dumb choices even though he vaguely remembers, from his previous times living his life, how annoying their consequences were....

Osokin iterates around again and again -- repeatedly living his life then getting the magician to send him back to the past to live his life over again -- each time failing to correct his stupid decisions.

But then, after the N'th iteration, he finally he achieves enough awareness that when he meets with the magician he realizes it's stupid to revisit his life again, without changing the nature of his mind -- and he agrees to join the magician's mystical sect and get tutored in the True Path....

Ouspensky's point of course is that normal humans don't have free will but basically live like deterministic machines pushed by their unconscious and emotions -- but if you join his Gurdjieff/Beelzebub sect, you can achieve real free will! (BIG DIGRESSION: Needless to say, I don't accept this philosophy, though I do find some germ of truth at the core of it. In my view, there is absolute freedom in the universe at a certain level -- the level Peirce called First -- and then there are patterns in the universe at another level -- the level Peirce called Third -- and there are subtle connections between First and Third, wherein some patterns seem to have more freedom associated with them than others.... It may well be that human-mind-patterns can achieve more freedom, in a sense, via practicing meditative and mystical disciplines like the ones Ouspensky preached -- though of course these practices can also lead to a bunch of delusions. But I don't believe that any practice can lead to a fundamental breaking-out from the world of determinism and delusion, which is pretty much what Ouspensky taught. It's a big exaggeration, unfortunately -- the only hope for breaking out of delusion altogether is to go totally beyond your human mind, which Ouspensky didn't really succeed in doing; he may have had awesome moments of insight, but he still remained human with all the beauty and flaws and screwiness implied thereby, blah blah blah....)

OK -- so Ouspensky's novel gave a funny twist on Nietzsche's Eternal Recurrence ... but it also seems somewhat relevant to these recent ideas about time travel.

The time-travel idea from the Greenberger and Svozil paper basically suggests that if we went back in time we'd find ourselves in the position of Ivan Osokin -- unable to make decisions other than the ones we're "postdestined" to make, so as to cause the future to come out as it's already known it's going to come out....

Ergo, the conclusions of Greenberger and Svozil hold up except in the presence of Ouspenskyan magicians!! ;-))

But another interesting possibility arises here. It may be that our present reality is not consistent with there having been time travelers going back into our past doing interesting stuff. However, sometime in the future there may be a time-travel-ful period full of time travelers cycling around and around -- and the world during that period may be whacky enough that the consistency of existence-of-meddling-time-travelers with observed reality is obvious...

Indeed, this is what I've often suspected. Once time travel is invented, maybe we'll be able to travel back in time fairly freely -- but only back to the point where time travel was invented -- not before. The Greenberger and Svozil results suggest that travel back before the invention of time travel may not be possible -- or may be possible only with very strict constraints -- because it can't be done too freely and still be done consistently with the world as it's known to be during that period (e.g. our period and our past). But once time travel is invented, free and whacky time travel from the future back till that point may well be consistent with the world after that point.

This suggests that the history of the universe may be divided into two periods: temporally forward and temporally bidirectional.

This is a fun vision of the post-Singularity world.... Post-Singularity may be post-temporality, in a sense. superhuman AI creates time machine, starts up the rampant-time-travel domain, and all heaven breaks loose ;-)

Yeah yeah, this is whacky speculation, I know. But it's not impossible according to known physics, and nor is it philosophically nonsensical.

The key point is that there may be consistent solutions of the universe's physics equations, according to which the universe at time T is consistent with time travellers from after T coming back and messing with the universe at time T in interesting ways that are obvious and noticeable to the folks living at time T.

The universe at our present time is consistent with time travelers from the future coming back and messing with our past, but not in ways dramatically noticeable by us. Of course, it's possible that time travelers did come back and mess with our past in ways that were important to us -- maybe that's the cause of the origin of life, the Big Bang, etc. -- these ideas have been explored in numerous science fiction novels. But even if so, this level of time-travel-based interference is pretty minimal compared to what may be possible in the post-Singularity period.

In some whacky, interesting but amateurish science fiction I wrote in the late 1980's (part of my never-finished meta-novel Wargasm), I described a character named Xaj Kalikak, who traveled into the future and practiced excessive time-travel until he'd revised the past and his own mind so many times that, in effect, the various loops of time-travel-induced-bidirectional-causation organized themselves into an intelligent mind. Instead of feedback loops of electricity in the brain, feedback loops of causation over time self-organized into a superintelligent mind. Perhaps this sort of thing will come true, and the superhuman mind following the Singularity will be transtemporal in a way we can't even imagine....